Thread forming tap with a non-circular cross-section and radially extending cutting edges

ABSTRACT

A thread-forming tap includes an elongated body having, at first end, a connector portion, and having at a second end, a thread-forming portion. The thread-forming portion includes at least one thread-forming helical thread exhibiting a non-circular cross-section having at least three lobes for plastically deforming an interior wall of a workpiece while forming an internal thread therein. A thread relief portion formed by a helical truncated thread, is disposed between the thread-forming thread and a terminal end of the body. Disposed at the end of the elongated body is at least one radially extending cutting edge for cutting parts of the workpiece interior wall.

TECHNICAL FIELD OF THE INVENTION

The thread 18 of the thread forming portion is non-circular (see FIG.2). Instead, each flank is provided with a pointed thread part, eachforming a lobe 30 for plastically deforming the interior wall of a holewhile forming an internal thread.

TECHNICAL BACKGROUND

Such a thread forming tap is known from EP-A-0 767 024 (corresponding toU.S. Pat. No. 5,797,710). That kind of tap is however disadvantageous,as it can barely be used for other openings than circular cylindricalholes.

SUMMARY OF THE INVENTION

The object of the invention is to achieve an improved thread formingtap.

This has been achieved by the tap of the initially defined kind, whereinat the open end of the elongated body, at least one radially extendingedge is provided for cutting parts of the interior wall of said opening.

Hereby is achieved a thread forming tap that pre-forms an opening to aform suitable for thread forming, which in turn extends the working lifeof the tap, since better thread forming conditions are achieved.

Preferably, said edge extends substantially from the central axis ofsaid elongated 30 body. Hereby, efficient forming of e.g. conical blindholes is achieved.

Suitably, a thread relief portion is provided between said threadforming portion and said open end, said relief portion being providedwith a thread having a cut ridge having a substantially circularcylindrical form and being divided by a helical groove for allowingturning the tap away from said opening. Hereby, easy removal of the tapafter thread forming is allowed.

Preferably, said cut ridge has a diameter smaller than the largestdiameter of the thread forming thread, and wherein said groove has adiameter slightly smaller than the diameter of the cut ridge.

Advantageously, the diameter of the thread of the thread forming portionincreases from the level of the cut ridges of the relief portion.Hereby, thread forming is made possible.

Suitably, at least one flank portion extending substantially radiallyfrom the axis of the body defines a flute extending substantially in alongitudinal direction of the body and through said thread formingportion, said thread forming thread thereby being cut into adiscontinuous thread. Hereby chip removal and supply of cooling fluidsupply is allowed.

In particular, said flute is substantially straight. Alternatively saidflute is helically shaped about the circumference of said body.

DRAWING SUMMARY

In the following, the invention will be described in more detail byreference to the accompanying drawings,

FIG. 1 illustrates a thread forming tap according to the invention,

FIG. 2 is a cross-section along the line II—II in FIG. 1,

FIG. 3 a is a perspective view of the tap shown in FIG. 1,

FIG. 3 b is an enlargement of the portion within the circle shown inFIG. 3 a,

FIG. 3 c is a front view of the tap shown in FIG. 1,

FIG. 4 illustrates an alternative thread forming tap.

DETAILED DESCRIPTION

FIG. 1 shows a thread forming tap 2, comprising an elongated body 4having an axis A—A. The elongated body 4 has at a first end 6, aconnector portion 8 and at a second end 10 a thread forming portion 12,and a thread relief portion 14. The tap 2 is preferably made of a highspeed steel or a solid carbide material. The connector portion 8 isadapted to be connected to a tool holder of a drilling or millingmachine.

The thread forming portion has a thread forming thread comprising aridge 18 separated by a helical groove 20. The diameter of the thread 18increases from the relief portion 14 towards the second end S and aftera few pitches, the diameter is constant (however not circular, seebelow).

The thread relief portion 14 is provided with a helical thread having acut (truncated) ridge 22 having a substantially circular cylindricalform separated by a helical groove 24. The cut ridge 22 has a smallerdiameter than the largest diameter of the thread forming thread 18 andthe groove 24 has a diameter slightly smaller than the diameter of thecut ridge 22. The pitch of the thread 22 and the groove 24 of the threadrelief portion 14 is the same as in the thread forming portion 12. Thethread forming portion allows for turning the tap away from said openingafter performed thread forming operation.

The threads 18 and 22 are separated in the longitudinal direction of thebody 4 by four flutes 26 (see also FIG. 2). The flutes 26 are thusdefined by four flank portions 28. At the periphery of each flankportion 28, the thread 18 and 22 is arranged, respectively (in FIG. 2,only the flank portions of the thread forming portion 12 can be seen).The purpose of the flutes 26 is to allow supply of a cooling fluid andto allow removal of chips.

The four flutes 26 divide the thread 18 into pieces of a thread, thepieces of thread thus forming a discontinuous or virtual thread. Thesame relates to the thread 22.

The thread 18 of the thread relief portion is non-circular (see FIG. 2).Instead, each flank is provided with a pointed thread part, each forminga lobe 30 for plastically deforming the interior wall of a hole whileforming an internal thread.

As can be better seen in FIGS. 3 a–3 c, the first end 10 defines aterminal open end 32 with four radially extending cutting edges 34 forcutting non-circular cylindrical holes. Cast blind holes are generallynot circular cylindrical, but may be slightly conical due to the castingprocess. The cutting edges 34 are thus utilized for forming thenon-circular cylindrical holes to a circular cylindrical shape

In FIG. 4, an alternative tap 4 is shown, the difference being that theflute 26 is somewhat helical, rather than straight.

OPERATION

A thread is to be formed in a through-hole or a blind hole of anon-circular cylindrica—in particular irregular—shape, respectively,e.g. in a molded piece of metal.

The tap 2 is moved to the hole defined by a wall (not shown). During itsintroduction into the hole, the tap is turned and the cutting edges 34cut the hole and form it to the predetermined circular cylindricalshape. The thread 18 enters the hole and the lobes 30 press the materialof the wall at such a high pressure that it is plastically deformed andfloates into the groove 20. A cooling fluid is introduced via theflutes. When the thread forming operation is finished, the tap is turnedin the opposite direction. The thread relief portion 14 facilitateswithdrawal of the tap from the hole.

Of course, the number of flanks 28 and flutes 26 may be less than four,e.g. three, and more than four, e.g. eight.

Furthermore, the flanks 28 and flutes 26 may be helically shaped.

Furthermore, the number of threads may be more than one, i.e. a pair ofparallel threads may be provided.

Furthermore, in FIG. 1, the shape of the thread relief portion isillustrated as circular cylindrical. However, it may instead be slightlyconical, the largest part of the cone being at the open end 32 of thebody 4.

Furthermore, the number of cutting edges may be less than four, e.g.one, or more than four, e.g. ten. However, it is advantageous to arrangethe same number of cutting edges as the number of flanks.

1. A thread-forming tap comprising an elongated body having a firstaxial end section and an opposite second axial end section; the firstend section including a connector portion; the second end section havinga thread-forming portion for forming a thread in an interior wall of aworkpiece; the thread-forming portion comprising at least onethread-forming thread separated by a first helical groove, saidthread-forming thread having a non-circular cross section including atleast three lobes adapted to physically deform the workpiece interiorwall while forming an internal thread therein; wherein the second endsection defines a terminal end at which there is disposed at least onegenerally radially extending cutting edge for cutting parts of theworkpiece interior wall; the second end section further including athread relief portion disposed between the thread-forming portion andthe terminal end, the thread relief portion comprising a thread in theform of a truncated ridge separated by a second helical groove; thetruncated ridge defining a circular cylinder.
 2. The thread-forming tapaccording to claim 1 wherein the at least one cutting edge comprises atleast three cutting edges.
 3. The thread-forming tap according to claim1 wherein the at least one cutting edge extends generally radiallyinwardly farther than the first helical groove.
 4. The thread-formingtap according to claim 1 wherein the at least one cutting edge is formedby a flute formed in an outer periphery of the body and extendinggenerally axially.
 5. The thread-forming tap according to claim 1wherein the truncated ridge has a largest diameter smaller than alargest diameter of the thread-forming thread, the second helical groovehaving a minimum diameter smaller than the maximum diameter of thetruncated ridge.
 6. The thread-forming tap according to claim 5 whereinthe thread-forming portion extends from an end of the truncated ridgeremote from the terminal end, an initial plurality of turns of thethread-forming portion increasing gradually in diameter in a directionaway from the truncated ridge.
 7. The thread-forming tap according toclaim 1 further including at least one flute formed in an outerperiphery of the body and extending generally axially through thethread-forming portion to define a discontinuity in the thread-formingportion.
 8. The thread-forming tap according to claim 7 wherein theflute is substantially straight.
 9. The thread-forming tap according toclaim 7 wherein the flute is helically shaped.
 10. A thread-forming tapcomprising an elongated body having a first axial end section and anopposite second axial end section; the first end section including aconnector portion; the second end section having a thread-formingportion for forming a thread in an interior wall of a workpiece; thethread-forming portion comprising at least one thread-forming threadseparated by a first helical groove, said thread-forming thread having anon-circular cross section including at least three lobes adapted tophysically deform the workpiece interior wall while forming an internalthread therein; the second end section further including a thread reliefportion disposed between the thread-forming portion and a terminal endof the second axial end section; the thread relief portion comprising athread in the form of a truncated ridge separated by a second helicalgroove, the truncated ridge defining a circular cylinder; wherein thethread-forming portion extends from an end of the truncated ridge remotefrom the terminal end, an initial plurality of turns of thethread-forming portion increasing gradually in diameter in a directionfrom the truncated ridge, there being at least one flute formed in anouter periphery of the body and extending generally axially through thethread-forming portion to define a discontinuity in the thread-formingportion, the at least one flute extending to the terminal end of thesecond end section and defining at such terminal end a cutting edgeextending generally radially for cutting parts of the workpieceinterior.